This invention relates to an anesthetic-vapor delivery apparatus for introducing anesthetic vapor into the breathing system of a patient undergoing surgery, and more particularly pertains to such an anesthetic-vapor delivery apparatus having an ambient-referenced pressure regulator toward more accurately controlling the rate of anesthetic agent delivery to the patent at any ambient atmospheric pressure.
Anesthetic-vapor delivery apparatuses of the prior art employ a source of carrier gas, typically a mixture of oxygen and nitrous oxide, connected via a carrier gas conduit to the patient's breathing system. A vaporizer chamber is provided for holding a liquid anesthetic agent in one portion thereof and in another portion for holding the evaporated anesthetic vapor. A vaporizer inlet port is typically connected to a proximal port in the carrier-gas conduit, and a vaporizer outlet port is connected to a distal port in the carrier-gas conduit so that a portion of the carrier-gas flowing from the source to the carrier-gas conduit is diverted into the vaporizer chamber where it mixes with the anesthetic vapor. This mixture exits the vaporizer chamber and re-enters through a distal port in the carrier-gas conduit to mix with the main stream of carrier gas. This last mixture of carrier gas and anesthetic vapor is delivered to the patient's breathing system.
The concentration of anesthetic in this mixture varies as the pressure at the patient's breathing system varies, because the breathing system pressure variations are reflected at the inlet port of the vaporizer chamber and thus in this entire parallel path including in the vaporizer chamber itself. The breathing system pressure that is reflected to the carrier-gas conduit may range from ambient to 2 p.s.i. above ambient (0 to 103 cm H.sub.2 O relative to ambient).
This problem has been ameliorated by adding a motor-driven positive displacement pump in the connection between the vaporizer outlet port and the distal port in the carrier-gas conduit, as is described in my U.S. Pat No. 4,770,168 issued Sep. 13, 1988. The positive displacement pump prevents changes in the breathing system from passing through the pipe connecting the distal port of the carrier-gas conduit to the vaporizer chamber.
However, pressure variations, though reduced, are still manifested in the vaporizer chamber and at the inlet of the pump, having been passed to the vaporizer chamber through the proximal conduit port.
The breathing-system variations in pressure that are reflected in the vaporizer chamber have the detrimental effect that the anesthetic-laden gas, with a given anesthetic concentration, is drawn into the positive displacement pump, and during a moment of pressure increase the rate of anesthetic delivery rises and during a moment of pressure decrease the rate of anesthetic delivery drops. Consequently the rate of anesthetic delivery is out of control to the degree that the pressure at the pump inlet varies.
It is an object of this invention to provide an anesthetic-vapor delivery apparatus in which the gas pressure at the inlet of the positive displacement pump is held at a pressure that is fixed differential pressure relative to the ambient pressure.
It is a further object of this invention to provide such an anesthetic-vapor delivery apparatus wherein the gas pressure at the inlet of the positive displacement pump is held at the ambient atmospheric pressure.
It is yet a further object of this invention to provide such an anesthetic-vapor delivery apparatus that controls the partial pressure of the anesthetic vapor in the gas delivered to the patient.